1. Field of the Invention
This invention relates to an amplitude limiter circuit for a transmission signal in the field of microwave communications, and more particularly to an amplitude limiter circuit to be used for a frequency-modulated wave or a phase-modulated wave in the microwave frequency band.
2. Prior Art
In general, a frequency-modulated wave or a phase-modulated wave (hereinafter referred to simply as an angle-modulated wave) received by a receiver in a radio communication system is often associated with an amplitude variation, and accordinly, for demodulation it is necessary to insert an amplitude limiter in the preceding stage of the demodulator thereby to preliminarily remove the amplitude variation.
Heretofore, as the above-referred type of amplitude limiter, is known a series diode type of amplitude limiter circuit, in which a pair of diodes connected in series with opposite polarities to each other achieve the amplitude limiting function while a current is passed from a constant current source through said diodes. This limiter circuit is constructed in such manner that electric currents in the forward direction are normally passed through both of said pair of diodes from a constant current source, thereby to automatically vary the series resistance values of the pair of diodes in response to an input signal level so as to suppress an output level variation. Thus, the pair of the diodes achieves the above-mentioned self-compensation effect. However, as the frequency of the input signal becomes higher, the influence of a phase-advanced current caused by a parallel capacitance of the diode cannot be neglected, resulting in an undesirable change of the phase of the output signal caused by the input signal level variation. Such an adverse effect is generally called "AM-PM conversion."